Dvc-coating with fully and partially stabilized zirconia

ABSTRACT

A dense vertical cracked microstructure in a ceramic layer system made of an underline partially stabilized zirconia layer and an above laying fully stabilized zirconia layer show good erosion resistance and long life time is provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to PCT Application No.PCT/EP2016/059828, having a filing date of May 3, 2016, based onEuropean Application No. 15172884.7, having a filing date of Jun. 19,2015, the entire contents both of which are hereby incorporated byreference.

FIELD OF TECHNOLOGY

The following relates to a ceramic layer-system with partially and fullystabilized zirconia which has also a dense vertical crackedmicrostructure (DVC).

BACKGROUND

Field feedback has shown that the current Thermal Barrier Coatings (TBC)of turbines suffer from issues related to:

1) Erosion: turbine blades with high porosity coatings containing alarge number of unmolten or semimolten particles show low erosionresistance. The development during the last years has pushed thermalspray coatings porosity upwards. However, that has caused the shrinkageof the spray ability window that allows coatings to receive highporosity and good cohesion. As a result, erosion has started manifestingitself as a major issue for coatings in specific parts and engines.2) Drilling damage: High porosity coatings contain less intimatecontacts between splats or splat and substrate and thus the requiredenergy for a crack to propagate is relatively low.This problem has been addressed by drilling before the coatingdeposition and reopening of the holes after coating deposition. Thisapproach minimizes the interaction between coating and laser and thatreduces significantly the coating delamination around the drilled holes.However, since each part has to be processed twice, this solution isassociated with longer drilling times that are reflected as increasedcost.3) Coating life: Thermal Spray porous coatings do not demonstrate at thesame level the high strain tolerance along the coating thickness whichcan be seen in other coating types such as EB-PVD.The thermal barrier coatings porosity has been increased to improvestrain tolerance. However as mentioned above, that can reduce the sprayability process window and influence negatively the cohesion and erosionresistance of the coatings.4) YSZ for TBC chemistries are currently limited to 1528° K maximumtemperature due to phase transformation issues. New chemistries havebeen adopted that present phase stability in higher temperatures.However they show significantly lower fracture toughness compared to thepartially stabilized zirconia and it is certain that their erosionresistance will be even less.

BRIEF DESCRIPTION

The FIGURE shows a DVC-coating with fully and partially stabilizedzirconia

DETAILED DESCRIPTION

The problems named under point 1 are addressed by adopting DenseVertical Cracked (DVC) coatings.

1) Erosion. DVC thermal barrier coatings have shown significantly lowerrates compared to their porous counterparts. That means for the samechemistry a porous coating will show more than 3× the erosion ratecompared to the DVC one.2) DVC coatings have increased cohesion and adhesion compared to thetypical porous coatings. The reason is that a very high ratio of fullymolten particles deposit on hot substrate or hot previously depositedsplats which promotes a good intimate bonding to develop between them.Improved adhesion requires high energy for a horizontal crack topropagate so that guarantees a lower delamination.3) Coating life. Due to the intimate contact between splats, the DVCcoatings show high fracture toughness along the parallel to thesubstrate plane. That, combined with their ability to accommodatethermal strain along the coating thickness due to their columnarmicrostructure ensures a high TBC life.4) DVC microstructures can be adopted on the new coating chemistries.That will create a bilayer DVC with partially stabilized zirconia as alower layer and fully stabilized zirconia as the upper layer. The lowerlayer will accommodate CTE mismatch with the bond coat and the TGO whilethe upper layer will provide the higher temperature capability.

The system consists of partially stabilized zirconia, especially 8YSZ asthe high fracture toughness lower layer to accommodate the CTE mismatchwith bond coat and TGO and a lower toughness upper layer of fullystabilized zirconia, especially 48YSZ to provide the high temperaturecapability.

Unlike other possible bilayer coating approaches, the similar chemistrybetween the two coatings enhances their bonding.

Appropriate preheating of the DVC PSZ will prepare its surface toreceive the fully molten particles of FSZ and due to the high localtemperatures during spraying allow diffusion between the two similarmaterials. Ideally a number of the vertical cracks will progress fromone coating to the other demonstrating the continuity between the twocoatings. In this manner the interface which has shown to be the weakestlink in other bi-layer systems will be reinforced.

The advantages that arise are:

1) The low fracture toughness of the FSZ with the adoption of a DVCmicrostructure will significantly increase. That will improve theerosion resistance of the coating.2) A good bonding between the two layers and as well with the bond coatwill increase the drilling damage tolerance. Less delamination will beobserved compared to other bilayer coating systems which have sufferedin the past from drilling.3) The columnar microstructure along the bilayer coating thickness willallow improved strain tolerance, thus increased coating life.4) Higher temperature capability compared to single layer DVC coatings.

The FIGURE shows a layer system 1.

The layer system 1 comprises a substrate 4 which is preferably metallicand very preferably made of a nickel or cobalt based super alloy.

On the substrate 4 a bond coat especially a metallic bond coat 7 andvery especially a NiCoCrAlY-based bond coat 7 is applied on.

On this bond coat 7 there is a thermally grown oxide (TGO, not shown)layer which is formed during further application of the ceramic layersor by an additional oxidation step or at least during use of the layersystem 1.

On the bond coat 7 there is applied a first zirconia layer 10 made of apartially stabilized zirconia.

The thickness of the partially stabilized zirconia layer 10 ispreferable between 75 μm-800 μm.

The porosity of the partially stabilized zirconia 10 is preferably lowerthan 5% and very preferably lower than 3%.

As an outer ceramic layer there is applied a fully stabilized zirconialayer 13, which is especially the outer most layer of the layer system1.

This outer layer can also be made of a pyrochlore ceramic, such asgadolinium zirconate (GZO), which partially or fully replaces the fullystabilized zirconia (FSZ).

The porosity of the fully stabilized zirconia 13 is lower than 5% andpreferably lower than 3%.

The thickness of the fully stabilized zirconia 13 is preferable between50 μm-800 μm.

The same parameters for thickness and porosity are also valid for thepyrochlore layer or pyrochlore/FSZ layer.

The stabilization in this zirconia based system can be reached by yttriaor by any other rare earth element as known by the state of the art orby a combination of that.

Preferably yttrium is used for stabilization.

In this layers 10, 13 there are cracks 16 present, which 19 are mostlypresent in the outer most layer 13 and preferably some of them 21 arepresent in both layers 10, 13.

Although the invention has been illustrated and described in greaterdetail with reference to the preferred exemplary embodiment, theinvention is not limited to the examples disclosed, and furthervariations can be inferred by a person skilled in the art, withoutdeparting from the scope of protection of the invention.

For the sake of clarity, it is to be understood that the use of “a” or“an” throughout this application does not exclude a plurality, and“comprising” does not exclude other steps or elements.

1. A ceramic layer system, at least comprising: a substrate made of a aninner partially stabilized zirconia layer and on it a fully stabilizedzirconia layer, wherein vertical cracks are present.
 2. The ceramiclayer system according to claim 1, wherein the fully stabilized zirconialayer is replaced partially or fully by a layer comprising or consistingof a pyrochlore material.
 3. The ceramic layer system according to claim1, wherein the cracks are only present in the fully stabilized zirconialayer or the outer layer with the pyrochlore material.
 4. The ceramiclayer system according to claim 1, wherein the cracks are present inboth ceramic layers.
 5. The ceramic layer according to claim 1, whereinthe porosity of the fully stabilized zirconia layer or the layer withthe pyrochlore material is lower than 5%.
 6. The ceramic layer systemaccording to claim 1, wherein the thickness of the partially stabilizedzirconia layer is between 75 μm-800 μm.
 7. The ceramic layer systemaccording to claim 1, wherein the thickness of the fully stabilizedzirconia layer or the layer with the pyrochlore material is between 50μm-800 μm.
 8. The ceramic layer system according to claim 1, wherein thezirconia or the zirconia layers are stabilized by yttria.
 9. The ceramiclayer system according to claim 1, wherein the porosity of the partiallystabilized zirconia layer is lower than 5%.
 10. The ceramic layer systemaccording to claim 1, wherein the partially stabilized zirconia isstabilized by yttria.
 11. The ceramic layer system according claim 1,wherein the substrate is a metallic substrate.
 12. The ceramic layersystem according claim 1, wherein the substrate has a metallic bond onthe substrate.
 13. The ceramic layer system according claim 1, whereinthe substrate is a NiCoCrAlY-based alloy.